This invention relates to pipe or conduit sections and more particularly, to lengths of reinforced plastic foam singlebore pipe that are joined together by a threading engagement and buried in the ground for use in transporting fluids, especially sewage.
In recent years, state and federal agencies have proposed and/or implemented regulations regarding tolerable leakage from underground sewage piping systems. Some organizations and municipal governments have also established standards regarding acceptable piping system head loss. Conventional piping systems for some types of sewage or liquid waste applications use clay, steel, or plastic pipe. Clay pipe has disadvantages in that close manufacturing tolerances are difficult to hold and leak-tight seals between adjacent pipe lengths are difficult to achieve. Some types of corrugated plastic and steel pipe do not have smooth interiors and thus offer an undesirable higher head loss. Thus, it is desirable to have a smooth interior on a strong pipe length that can be joined relatively easily and quickly with an adjacent pipe length to form a leak-tight seal.
It is known to join metal pipe or conduit by threaded end engagement. Examples of this are a U.S. Pat. to Barnstead No. 836,173 and a U.S. Pat. to Williams No. 3,794,364. Recently, lengths of plastic pipe have been disclosed which make use of integral reinforcing ribs or threaded end connections. Examples of these are the U.S. Pat. to French No. 3,813,115 and the U.S. Pat. to Glover, No. 3,791,679.
It has been found that, under certain circumstances, the use of these types of pipe is not entirely without disadvantages. A number of problems have been encountered.
First, due to ground settling beneath the pipe and shifting of pipe or conduit during back-filling, void areas beneath the pipe or conduit, particularly at the joint, have occurred. Thus, there is a need for the length of pipe or conduit to be structurally rigid and able to withstand the imposed bending and twisting forces. It is important that the joint region be strong and able to withstand these forces also.
Second, in the case of threaded joints, it is desirable that the installers of the piping system have a visual indication that adjacent pipe lengths or conduit lengths have been fully threadingly engaged.
Third, when laying pipe or conduit in an open trench, it is desirable that the lengths of pipe or conduit be essentially supported along their bottom points of contact in a substantially uniform manner to prevent canting or undue stress or an unsupported portion of the length of pipe or conduit.
Fourth, it is desirable to fabricate plastic pipe or conduit with a minimum use of plastic material. This objective can be achieved with novel design by (1) providing the pipe or conduit wall with reinforcing grid work to enable smaller wall thicknesses to be used, and (2) providing a thread design that conserves plastic material while still providing an effective and strong threaded engagement.